Prior art sonar signal processing has utilized several techniques to process the sonar signal to provide the desired detection, enhancement and classification functions. Such prior art techniques have included a system in which the sonar signal is time-segmented, with each time-segment then being digitized, frequency analyzed, converted to a power-vs.-frequency input spectrum and then inspected for persistent features which persistent features are assumed to be the result of naval-vessel-originated sound waves. The resulting persistent features or signal patterns are typically displayed upon a CRT display system for visual analysis.
One technique has been to threshold successive input spectra as derived from the contiguous or successive time-segments. This technique permits the processing operation to follow the persistent features of each input spectrum as they vary in frequency and to provide the resulting line-sets from the successive input spectra as frequency-varying lines on a display device. However, thresholding to delete the obfuscating noise signals must be set at a high level whereby certain persistent features may be deleted along with the noise unless such persistent features have a power level much higher than that of the average noise power level. Additionally, such technique does not take advantage of the fact that such persistent features may occur in the successive input spectra, or have counterparts across the spectrum, whereby the persistent features could be enhanced with intensity with respect to the noise signal.
Another technique has been to perform Automatic Line Integration (ALI) upon the input spectra. This ALE process consists of dividing each of the input spectra into a plurality of like frequency bins and then integrating the power over the spectra of the like time-segments for each bin. However, such technique does not permit the processing operation to follow the persistent features of each input spectrum if the persistent features vary in frequency over a number of successive input spectra. Thus, in those input spectra having sever line-instability, i.e., whether persistent features of successive input spectra vary in frequency with respect to each other, such ALI technique degrades signal enhancement and further is unable to provide much information on any line-instability that may be present in the input spectra.